Cosmology and extragalactic astronomy Mat Page Mullard Space Science Lab, UCL 2. Galaxies

1. Galaxies This lecture: Classification of galaxies –Spirals –Ellipticals –Irregulars Characteristics of the different types Slide 2

Hubble’s tuning fork Basic set of galaxy types. Slide 3

Ellipticals Spirals –barred –normal Irregulars 3 groups: Slide 4

Classified as E0-E7 En n=10(1-b/a) Ellipticals M87: Giant elliptical (E1) Classified according to view from Earth! Slide 5

Most of the galaxies in the local group are dwarf ellipticals! M 110: dwarf elliptical (E6) Ellipticals Slide 6

Elliptical characteristics Random star motions Little dust, little gas –gas is hot, can be X-ray sources no star formation no hot, young bright stars old, red stars no spiral structure Large range of sizes: M o. Slide 7

Spirals Like the Milky Way –central bulge –flattened disc –spiral pattern –sometimes a bar Classified as S (for spiral) or SB (barred spiral) plus a,b,c,(d) NGC 1365 SBbc Slide 8 Credit: Marco Iacobelli (XMM SOC) & ESA XMM-Newton Optical Monitor

NGC 2997: Sc This one has no ‘B’ because it has no bar Slide 9

Spiral classification Sa and SBa have: large central bulges Tightly-wound spiral arms Sc and SBc have: small central bulges Loosely-wound spiral arms M81(Sab) NGC4321(Sc) Slide 10

M81 UV (XMM)) Slide 11

M81 21cm Slide 12

M100 (AAT)) Slide 13

M51 (HST)) Slide 14

Characteristics of spirals Consistent rotational motion in the disc Lots of gas and dust in the disc Star formation in disc, especially in the arms Young, bright blue stars in the arms Spiral structure x10 11 M o Slide 15

Lenticular galaxies Lenticular -- ‘lens-like’ Like a spiral, but without spiral structure Designated S0 or SB0 I think difficult to tell from ellipticals by eye. Slide 16

Irregulars No definite structure –Everything that doesn’t fit on the tuning fork Mostly small, faint May have strong star formation May have lots of gas and dust –e.g. LMC and SMC Slide 17

LMC in the UV (Swift UVOT)) Slide 18

Back to Hubble’s tuning fork Hubble thought that galaxies evolved from ellipticals to spirals This is very unlikely! Slide 19

Some key points about galaxies: Ellipticals –no overall rotation –old stars, little gas. Spirals –rotating disc –old stars in the bulge –young stars in the disc and in bulge Lenticulars –bulge and disc without spiral structure Irregulars –often lots of gas –sometimes strong star formation Slide 20